The present invention relates to video processing apparatus of the type for producing a wipe wherein a decreasing area of successive output frames is derived from a first video signal and an increasing proportion of said area is derived from a second video input signal. The video effect known as a "wipe" consists of gradually removing a first image to reveal a second image; as if the first image were being rubbed off, or wiped from, the first image. Analogue systems for producing wipes are well known, for example, as shown in U.S. Pat. No. 3,371,160. The advent of operational amplifiers allowed analogue wipe techniques to be improved, for example, as shown in the article by Michael Cox published under the title "Electronic Effects" in the June 1982 edition of the British Kinematography Sound and Television Society Journal. Cox describes a system in which two video input signals (A and B) are combined by a control voltage C produced by a pattern generator. The instantaneous output is given by AC +B (1-C) where C has a value between 0 and 1. The formation of the control signal is shown in which a sawtooth waveform is generated having a rise time equal to the duration of one TV line period. Manual potentiometers (referred to as faders) are then provided to adjust the position and size to a window over which C changes from 0 to 1. Similar means are then provided to generate a ramp equal to the frame period thus allowing wipes to be controlled in both the horizontal and vertical directions.
The disclosure states that it is possible to generate almost any immagenable pattern, using mixtures of analogue and digital circuit techniques. It can therefore be appreciated that such generation would not be performed by an operator but by a skilled electronic engineer. Thus the techniques could be employed when designing a machine but, once built, the operator would be presented with a collection of preset wipe shapes with the facility for creating new shapes.
It is an object of the invention to provide an improved system for wiping between video images. It is a further object of the invention to provide a versatiile system in which an operator may define the wipe shape. A further object of the invention is to provide a system for receiving two digitally encoded video signals to produce an output signal which wipes between the input signals.